._--_Nautilus_, like the other Cephalopoda, exhibits a
great concentration of the typical Molluscan ganglia, as shown in fig.
13. The ganglia take on a band-like form, and are but little
differentiated from their commissures and connectives--an archaic
condition reminding us of _Chiton_. The special optic outgrowth of the
cerebral ganglion, the optical ganglion (fig. 13, o), is
characteristic. The cerebral ganglion-pair (a) lying above the
oesophagus is connected with two suboesophageal ganglion-pairs, of
band-like form. The anterior of these is the pedal _b, b_, and
supplies the circumoral lobes and tentacles, and the funnel, a fact
which proves the pedal origin of these organs. The hinder band is the
visceral and pleural pair fused; from its pleural portion nerves pass
to the mantle, from its visceral portion nerves to the branchiae and
genital ganglion (fig. 13, d), and in immediate connexion with the
latter is a nerve to the osphradium or olfactory papilla. A labial
commissure arises by a double root from the cerebral ganglia and gives
off a stomatogastric commissure, which passes under the pharynx
immediately behind the radula and bears a buccal ganglion on either
side.

_Special Sense-Organs._--_Nautilus_ possesses a pair of osphradial
papillae (fig. 4, olf) corresponding in position and innervation to
Spengel's organ placed at the base of the ctenidia (branchiae) in all
classes of Mollusca. This organ has not been detected in other
Cephalopoda. _Nautilus_ possesses other olfactory organs in the region
of the head. Just below the eye is a small triangular process (not
seen in our figures), having the structure of a shortened and
highly-modified tentacle and sheath. By A. Valenciennes, who is
followed by W. Keferstein, this is regarded as an olfactory organ. The
large nerve which runs to this organ originates from the point of
juncture of the pedal with the optic ganglion. The lamelliform organ
upon the inner inferior tentacular lobe of _Nautilus_ is possibly also
olfactory in function. In Dibranchs behind the eye is a pit or open
canal supplied by a nerve corresponding in origin to the olfactory
nerve of _Nautilus_ above mentioned. Possibly the sense of taste
resides in certain processes within the mouth of _Nautilus_ and other
Cephalopoda.

The otocysts of _Nautilus_ were discovered by J.D. Macdonald. Each
lies at the side of the head, ve